Why do we celebrate Shivaratri?

Maha Shivratri or Shivaratri (Night of Shiva or "Great Night of Shiva") is a festival celebrated every year on the 13th day/13th night (breaking fast/'vrata' on 14th after sunrise) in the Krishna Paksha (waning moon) of the month of Maagha(as per Shalivahana or Gujarati Vikrama) or Phalguna (as per Vikrama) in the Hindu Calendar (that is, the night before and day of the new moon). The festival is principally celebrated by offerings of Bael (Bilva) leaves to Lord Shiva, all day fasting and an all night long vigil. Per scriptural and discipleship traditions, the penances are performed in order to gain boons in the practice of Yoga and meditation, in order to reach the goal more swiftly and avoid rebirth.

After creation was complete, Parvati asked Lord Shiva which rituals pleased him the most. The Lord replied that the 13th night of the new moon, during the month of Maagha, is his most favourite day. Parvati repeated these words to her friends, from whom the word spread over all creation.

This year we celebrate Shivaratri on the 12th February in our Center.
Last year Guruji explained the meaning of this Holy festival dedicated to Lord Shiva, who is the destroyer (of ignorance etc.)
Sri Swami Vishwananda: Jai Gurudev!

Audience: Jai Gurudev!

Sri Swami Vishwananda: Firstly welcome to everybody!
As you know today I was explaining before Shivaratri, which is one of the main Hindu festival. How many of you know what is Shivaratri? (Audience laughing but keeps silent)

Sri Swami Vishwananda: (Smiling) Lift your hand. (Audience laughing)
Go on. (Audience laughing, murmuring) Eh?

Someone: Trying to transcend the mind.

Sri Swami Vishwananda: Good.
What else?

Someone: Shiva can sleep tonight.

Sri Swami Vishwananda: Eh?

Someone(repeating) Shiva can sleep tonight.

Sri Swami Vishwananda (smiling) No, actually He can’t sleep tonight. (Audience laughing)
We will keep Him awake all the time, by doing the four time Prayer. (Smiling)
Go on.

Someone: Marriage.
It is the union between Shiva and Shakti. Shiva and Parvati.

Sri Swami Vishwananda: No, actually it’s not today. It’s not on Shivaratri.
What is the Original of Shivaratri?

Someone: There are different meanings.

Sri Swami Vishwananda: There are different meanings of course. There are different stories that link to Shiva on that day.
But the most important why we celebrate Shivararti is…

Someone (whispering) surrender

Sri Swami Vishwananda: … to show the love we have for mankind.

A nice video which puts one in the right mood for Shivaratri :
http://www.youtube.com/watch?v=1T41bNjSm_4&feature=related